The present invention relates to a portable radio, such as primarily a digital portable telephone having a built-in plane antenna, and more particularly to the space-saving structure of the built-in antenna. Further, the invention relates to a plane antenna which is mainly used in a digital portable radio (a portable telephone).
A plane antenna has been widely used as a built-in antenna of a digital portable telephone because of its advantages of compactness and high gain. The built-in antenna of the portable telephone is directly mounted on a circuit substrate.
In recent years, a portable telephone market has spread into wide use, and the portable telephone is miniaturized so as to sufficiently fit in a pocket. The portable telephone tends to be miniaturized further. In conjunction of miniaturization of the portable telephone, there is also a demand for a reduction in the space occupied by the built-in antenna attached to the portable telephone.
However, if the size of the portable telephone is reduced further, it becomes difficult to ensure space for the built-in antenna. This is because as the size of the case is reduced, a circuit originally made up of a plurality of circuit boards is constituted of only one board. In addition, the circuit board itself becomes more compact. For these reasons, the electronic components are mounted on the circuit board in high density. Accordingly, the space required to mount the built-in antenna on the board is reduced, the antenna must be formed in small size. In general, a small-sized antenna entails the deterioration of gain, which in turn may result in a drop in the performance of the portable telephone. However, if an attempt is made to increase the size of the built-in antenna, it becomes impossible to store an electronic circuit in the case.
Aside from the foregoing problem, there is a lot of talk about the problem related to a line. More specifically, the frequency of utilization of a line is increased in proportion to an increase in the number of portable telephones, and is traffic becomes nearly full. The number of circuit lines is increased by utilization of a frequency range of the analog portable telephone as measures against the increase in the traffic. However, this method requires two receiving ranges. It is impossible to provide an existing built-in antenna having a narrow range with the sensitivity which covers two ranges. Therefore, there is needed an antenna which produces two resonance ranges (or multiple resonance) and has sensitivity over two ranges.
To solve the foregoing problem and to minimize the area occupied by a board of a built-in antenna without deterioration of a gain, a radiation element of a built-in antenna is mounted on an internal wall of a case, and a conductor is provided which is capable of electrically connecting the radiation element to a power feed circuit provided on the board, thereby providing a built-in antenna which achieves high gain and requires the space occupied by the circuit board.
To ensure sensitivity over to receiving ranges, an antenna radiation element is provided with a branch where one end of a longer radiation element is connected to one end of a shorter radiation element. The other ends of the longer and shorter radiation elements are open ends and are positioned in close proximity to each other so as to form gap between them. Impedance matching is accomplished by means of the mutual coupling capacitance developed in the gap, and power is fed to the branch.